1. Field of the Invention
This invention relates to illumination systems for light microscopes utilizing objective lens means and more particularly to such systems that utilize illuminating light directed at an oblique angle relative to the optical axis of the objective lens.
2. The Prior Art
The use with microscopes of what is commonly referred to as "oblique light" was of interest toward the end of the last century and the beginning of this, but the many devices designed for that purpose, although ingenious in some cases, have failed to survive. See The Intelligent Use of the Microscope, Oliver, C. W., Chemical Publishing Co., 1953.
Oliver carefully limits his meaning of "oblique light" to the "use of a narrow cone or beam of rays directed upon the object from any direction other than the optical axis provided it enters the object glass." Ibid. at 94. In this way he excludes from his discussion those systems that use rays directed onto a specimen from a direction other than the optical axis but which do not enter the object glass, as well as systems where the light does not enter the objective lens at an angle (such as systems that merely tilt the specimen stage). Illumination provided by systems in which the primary beam does not enter the objective is generally known and commonly referred to as "dark field" illumination as more fully discussed in Photomicrography a Comprehensive Treatise, Loveland, R. P., John Weily & Sons, Chapter 12. Although the present invention utilizes true oblique lighting as that term is used by Oliver, and is thereby clearly distinguishable from "dark field" systems, a brief description of "bright field" and "dark field" illumination will help to differentiate and more fully highlight the attributes of the present invention.
Illumination systems that direct rays onto a specimen along the optical axis create "bright field" illumination, so named because the rays passing through the field surrounding the specimen and entering the microscope objective are unimpeded and thus bright compared to the rays attenuated by passing through the specimen. In a "dark field" system, the relative brightness is reversed by directing only light rays onto the specimen field which are angled relative to the optical axis and directed to fall outside the objective aperture. All of the light passing through the specimen field surrounding the specimen is unimpeded and thus does not enter and is therefore not "seen" by the objective. Some of the light directed onto the specimen will be scattered, however, into secondary light rays, some of which will enter the objective (and be "seen"). Thus, the object appears brighter than the surrounding dark field. Such a system is described in U.S. Pat. No. 4,896,966.
The prior art contains a number of systems that combine "bright field" and "dark field" illumination for use both together and selectively, as illustrated in U.K. Pat. No. 887,230, and U.S. Pat. No. 4,601,551. In all of these systems the primary illuminating light is either aligned with the optical axis or angled to fall outside of the objective aperture.
The invention of U.S. Pat. No. 3,876,283, teaches the use of a system which uses true oblique lighting, by use of a prism located on the optical axis of a microscope condenser to laterally off-set an axial illumination beam to a path separate from the optical axis so as to direct the beam onto an off center location on the condenser lens. When a light beam parallel to the optical axis enters an off center location on a condenser lens, the beam will exit the lens at an angle to the optical axis. The degree of the angle is a function of the displacement of the beam from the center of the lens. When, as in patent '283, the angle is within the objective aperture, the system produces true oblique lighting as defined by Oliver (the light is "seen" by the objective). In order to achieve the maximum oblique angle for the beam, it must exit the condenser lens at or very near its periphery at an angle that is just within the objective aperture. While the teachings of patent '283 make this possible (by adding a wedge-shaped prism to the plano prism shown), each different condenser and objective combination will require a different pair of prisms to achieve a maximum angle. Otherwise, depending on the characteristics of the objective lens and condenser lens being used, it may be necessary, with the system of patent '283, to direct the laterally off-set beam onto the condenser lens at a location inwardly of its periphery in order to have the resultant exit angle within the objective aperture. In such cases, the maximum possible oblique angle will not be realized and, as will be explained below, the maximum resolution power of the system will not be achieved.
In patent '283, the location of the illuminating beam (between 15 and 17) and beam path shifting means 23 (prism) on the optical axis limits the system by permitting the use of only a single illumination beam. The references cited above are typical of the prior art in that they fail to recognize the real potential of oblique lighting to enhance resolution. Patent '283, in fact, does not acknowledge the resolution enhancing potential of oblique light, but instead cites as a reason for its use the casting of shadows to highlight uneven areas of the specimen. It is not, therefore, necessarily an object or desideratum of patent '283 to provide a maximum oblique angle (for example, too much shadowing might obscure details). But, one of the requirements of realizing the full potential of oblique lighting to dramatically enhance resolution is that the angle of the oblique light be maximized. For a single beam system, maximum resolution is achieved for a given condenser lens/objective lens combination by having the illumination beam exit the condenser lens' periphery so that the light illuminating the object is at a maximum oblique angle and still within the objective aperture. By making it possible to adjust the angle at which the beam exits the condenser lens independently of the location where it exits, the angle of the light (relative to the optical axis of the objective lens means) illuminating the specimen can be fully maximized. Likewise, by being able to adjust the location where the beam exits the condenser independently of the angle at which it exits, any condenser can be used to its fullest potential. With the ability to so adjust the angle and location of the beam exiting the condenser lens, a large condenser lens (high numerical aperture) can be used to achieve maximum oblique lighting for most objective lenses.
The present invention teaches that the essential requirement for realizing the maximum potential of true oblique lighting is the ability to direct two or more separate and distinct light beams onto the objective wherein each beam is at the maximum angle to the objective axis that permits the illumination to enter the objective. This, of physical necessity, requires that the beam shifting means be located off the optical axis of the objective. Also, the present invention overcomes the anisotropy that is found in prior art oblique illuminating systems.
In addition, the present invention teaches a real time, 3-D system using multiple beams, which goes far beyond what can be achieved with a single beam, such as that described in U.S. Pat. No. 4,072,967. Patent '967 teaches how to achieve a 3-D image using a microscope with a single condenser lens and a single objective lens, by placing complimentary filters across the left and right halves of the condenser lens and placing a complementary filter set in the binocular eyepieces. With this type of system, the degree of parallax is fixed. Furthermore, there is very little disparity in parallax between the left and right images, especially at the center of the image field. In contrast, with the present invention, the left and right images are independently controlled and the degree of parallax between them can be easily adjusted to match the type of objective being employed and the type of specimen being viewed. In addition, there is another and possibly even more important advantage with the present invention, which is the ability to achieve a greater depth of field without loss of resolution, as is more fully explained below. This is a critical prerequisite for producing a sharp 3-D image.